The invention relates to the field of optical fibers.
Several methods have been developed for covering the core and cladding material of an optical fiber in a coating material to confer specific properties, for example robustness, flexibility, or protection against hostile environments.
Amongst those methods, proposals have been made in particular for a covering constituted by a first coating portion of photocurable resin having a low modulus of elasticity, generally having an acrylic base, and by a second coating portion having a high modulus of elasticity, generally being of the same kind as the first coating portion, and being intimately bonded therewith.
These photocurable resins present fast reaction kinetics in cross-linking, enabling speeds of about 2000 meters per minute (m/min) to be obtained. Nevertheless, such photocurable resins provide only a relatively narrow range of specific properties. In addition, since the resins need to be deposited by being applied at relatively low temperatures, typically 90° C., and since the temperature at which optical fibers are drawn is about 1600° C., it is imperative for the uncoated optical fiber to cool down considerably on leaving the fiber-drawing furnace, or in other words for the distance between the fiber-drawing furnace and the coating apparatus to be large. Finally, the cost of such photocurable resins is high.
Proposals have also been made to apply an additional coating, referred to as a “buffer” coating, to optical fibers already provided with identical first and second coating portions, the buffer coating being constituted by an extruded thermotropic liquid crystal polymer (TLCP). Such an optical fiber is described in particular in U.S. Pat. Nos. 4,778,244 and 4,906,066. That type of coating (or covering) enables certain specific properties of optical fibers to be reinforced, and in particular their ability to withstand external stresses induced by conditions during installation and/or use. However that is not sufficient, particularly given the wide variety of properties or characteristics that are required in numerous applications for optical fibers. In addition, the operation of adding the buffer coating takes place at a speed that is very slow, typically about 1 m/min to 10 m/min, which is very far from the usual speed at which optical fiber is manufactured.